The present invention relates to an electrodeposition coating composition that is excellent in bath stability and that can give a cured film enhanced in hardness and to an electrodeposition coating process.
Electrodeposition coating is a process by which even substrates complicated in form can be uniformly coated and by which coating can be conducted automatically and continuously. Therefore, it has been generally used as a method for undercoating automotive bodies and like large-sized substrates complicated in shape and required to have highly corrosion resistance. As compared with other coating processes, it is very highly efficient from the paint consumption viewpoint, hence economical. It has thus come into wide use as an industrial painting process.
The electrodeposition coating compositions currently used in such electrodeposition coating processes are generally based on isocyanate curing system. Those electrodeposition coating compositions which utilize said isocyanate curing system can give cured films having good hardness and provide excellent corrosion resistance. However, isocyanates are very highly reactive and, when used as they are, worsen bath stability. Therefore, they are generally used in the form blocked with alcohols or the like.
Meanwhile, when such blocked isocyanates are used in coating compositions, the blocking agents are released in the step of baking. They dissipate and form gum, soot or the like, contaminating the environment inside and outside heaters and drying ovens, causing deterioration of deodorizing catalysts and harming the working environment.
In addition, the isocyanates formed upon deblocking are themselves toxic, posing an environment compatibility problem. Furthermore, for forming superior coatings in the isocyanate curing system, it is necessary to raise its curing temperature. This means heavy consumption of energy in the step of baking.
To solve these problems, attempts have been made to develop electrodeposition coating compositions which can substitute for the isocyanate curing system, i.e. electrodeposition coating compositions utilizing novel curing systems, for example curing systems based on the condensation reaction of melamine or phenol resins, Michael addition polymerization curing systems, oxidative polymerization curing systems and transesterification reaction curing systems.
Like the isocyanate curing system-based ones, those electrodeposition coating compositions which have so far been proposed and in which novel curing systems are utilized invariably give, after film formation by electrode-position, cured films by utilizing external energy, for example heat provided by baking/drying ovens, in the curing reaction.
In such systems, the main curing reaction, curing catalyst activation reaction and other reactions for producing cured films having desired characteristics can proceed when the threshold of energy required for the reactions is crossed by heating. In that case, these novel curing systems are all constituted by incorporating beforehand all elements required for constituting said curing systems into the respective coating compositions. Therefore, the curing reactions in the electrodeposition coating compositions in which these novel curing systems are utilized can be started only by heating, whether there is the step of electrodeposition or not. Thus, they are based on the so-called heat potentiality.
Therefore, the electrodeposition coating compositions utilizing said novel curing systems, which are based on the heat potentiality mentioned above, cannot avoid the progress of curing reactions in the bath as caused, for instance, by Joule""s heat generated in the electrodeposition vessel upon voltage application. They are thus all poor in bath stability.
On the other hand, for improving the bath stability, it is necessary to widen the difference between bath temperature and baking temperature as far as possible and it is also necessary to raise the curing temperature itself of the electrodeposition coating composition employed so that the successive curing reactions in the bath can be inhibited. The latter means that the baking temperatures so far used give only insufficient hardness of cured films.
In particular, those electrodeposition coating compositions in which oxidative polymerization or transesterification curing systems are used have a problem in that the bath stability and curability can hardly be balanced. Therefore, it is very difficult to put to practical use the electrodeposition coating compositions in which such curing systems as mentioned above are utilized.
In view of the above-mentioned state of the art, it is a primary object of the present invention to provide an electrodeposition coating composition and an electrodeposition coating process with or by which excellent bath stability can be attained and the hardness of cured films obtained can be improved.
As a result of their intensive investigations concerning the processes of electrodeposition coating, the present inventors found that bath stability and film hardness can be made consistent when the voltage application process, which is utilized exclusively in film deposition in conventional electrodeposition coating processes, is made to be positively involved in the curing reaction of films and that reduction in curing temperature can also be readily attained. Based on such findings, the present invention has now been completed.
Thus, the present invention consists in an electrodeposition coating composition to be used in an electrodeposition coating process which comprises the electrodeposition step of immersing a substrate in an electrodeposition coating composition and applying a voltage between the substrate as an anode or cathode and a counter electrode to thereby form a film of said electrodeposition coating composition on the surface of the substrate, and the heating step of heating said film obtained in the above electrodeposition step, and said electrodeposition coating composition comprising having electrodeposition potentiality against the curing reaction of said film.
The present invention also provides an electrodeposition coating process which comprises using the above-mentioned electrodeposition coating composition.